1. Field of the Invention
The invention relates to plasma generating systems and is particularly applicable to systems which utilize plasma for processing a substrate such as a semiconductor wafer.
2. Discussion of the Background
In many electrical devices or solid state manufacturing processes, a plasma reacts with a substrate such as a semiconductor wafer. Plasma processing systems use, for example, radio-frequency (RF) power to initiate and sustain the plasma, with the RF power sent into a gas by an inductive and/or capacitive plasma coupling element. By way of example, as shown in FIG. 1A, a conductive loop or helical coil 5 can serve as an inductive coupling element. As shown in FIG. 1B, an electrode, or a pair of electrodes 5E1 and 5E2, can serve as capacitive coupling elements.
In order to generate a plasma, several steps are required. First, as illustrated in FIGS. 1A and 1B, as is supplied to a process chamber 1 through gas inlets 6. An RF power source 3, with an output impedance R.sub.s, supplies RF power to a plasma coupling element (e.g., coil 5, in FIG. 1A, or electrodes 5E1 and 5E2 in FIG. 1B) which, in turn, excites the gas into a plasma within a certain region (plasma region 2) of the process chamber. The plasma is then used to process a substrate, e.g., a semiconductor wafer 40. Many conventional systems supply the RF power through an electrical matching network. U.S. Pat. No. 5.325.019, to Miller & Kamon, has presented a method of using information at the frequencies to monitor or assess the state of a plasma. See also Turner et al. (U.S. Pat. No. 5.576,629). Gesche & Vey (U.S. Pat. No. 5.025,135) and Patrick et al. (U.S. Pat. No. 5,474.648).
A plasma is often sustained in a portion of its current-voltage characteristic known as the "abnormal glow regime," as represented in FIG. 3. In this regime, since a high density of electrons and ions are present, and since significant electric fields are also present, the plasma is susceptible to arcing, which typically occurs in a portion of the current-voltage characteristic which is adjacent the abnormal glow regime. Arcing is a condition in which the region of current flow in a plasma, normally spread over a significant volume, collapses into a highly localized region (called an arcing region) which contains a concentrated arcing current. During arcing, due to the high concentration of power dissipation and the high speeds attained by electrons and ions in the arcing region, surfaces of the substrate or the system components can be altered or damaged from ion or electron implantation, from sputtering of the surfaces and/or localized heating (which can cause spalling).
Although low-severity, occasional arcing which causes little or no damage occurs during normal operation of a plasma processing system, high-severity or more frequent arcing can be a significant problem, which can cause inferior performance, or even failure, the circuits being processed. Severe arcing can also damage one or more components of the processing system, such that expensive components must be replaced. Furthermore, the processing system must be shut down to replace the damaged components and/or to correct the arcing problem. Even if components in the system are not damaged enough to require immediate replacement, pitting of the surfaces of the chamber, electrodes, or other components can cause particulates, which contaminate the system or the substrate. In addition, in systems which use an electrostatic chuck, arcing can disrupt the electric fields that clamp the wafer to the chuck, thereby causing the wafer to become unclamped or decoupled from the chuck.
Although high-severity arcing is sometimes visible as a flash of light, if the arcing is severe enough to see, the system or substrate has likely already been damaged. Moreover, low or moderately severe arcing (which can be a precursor to more severe arcs) are often difficult to detect. In addition, arcing can occur for various reasons including, e.g., excessive power or localized impurities/contaminants accumulating on one or more components (or the substrate) within the plasma process system. Further, once arcing has occurred, subsequent, potentially more severe arcing is more likely.
Due to the difficultly in predicting arcing, conventional systems are sometimes run at a relatively low or safe power in order to avoid conditions which could lead to arcing. However, some processes preclude, or are not optimally run, using conservative power levels. In addition, even the practice of using RF power conservatively is not always effective, since the safe operating range cannot always be determined, and since arcing can occur for various reasons. The difficulty in determining the safe operating range is exacerbated by the tendency of systems to become increasingly susceptible to arcing as they become contaminated and degraded from normal use.
One of the first patents to describe a method of detecting an arc within a glow discharge is Oppel (U.S. Pat. No. 4,193,070). Therein, a control system monitors the discharge voltage and current. When the voltage falls below a threshold value and the current rises above a threshold value, an arc is said to have occurred. Several others have followed a similar approach to monitoring the discharge voltage and/or current, their rates of change, and/or any such random anomaly in an electrical signal. See Teschner (U.S. Pat. No. 5.192,894), Anderson et al. (U.S. Pat. No. 5.241,152), Drummond (U.S. Pat. No. 5,427,669), and Maass (U.S. Pat. No. 5.611,899 (hereinafter "the '899 patent.")) Notably, the '899 patent also presents a method of "severe" arc prevention wherein the AC power supply is adjusted to avoid an arc after sensing several defined conditions. A large arc can then possibly be construed as one which damages a substrate and halts a process. Yet, the method of sensing an arc is limited to monitoring the voltage of a plasma coupling element and its rate of change.
Neural networks have been used for both predication and control in many areas. A use of neural networks in semiconductor processing to predict the endpoint of an etch process is discussed by Maynard et al. in "Plasma etching endpointing by monitoring RF power systems with an artificial neural network," Electrochem. Soc. Proc., 95-4, p189-207, 1995, and "Plasma etching endpointing by monitoring radio-frequency power systems with an artificial neural network," J. Electrochem. Soc., 143(6). The system monitor was trained to observe several electrical components, in particular, transmitted power, reflected power, capacitor values in the match network, de bias, etc., and correlate their behavior with the etch process endpoint wherein the endpoint was determined and programmed via an ellipsometer and user input, respectively. In essence, the neural network presents a means of correlating the inter-relationships of several measurable variables with the occurrence of an event in order to establish predictability.
One use of a neural network is the characterization of a system. The literature that presents attempts to characterize the electrical properties is numerous; see Logan, Mazza & Davidse, "Electrical characterization of radio-frequency sputtering gas discharge," J.Vac. Sci Technol., 6, p. 120 (1968); Godyak, "Electrical characteristics of parallel-plate RF discharges in Argon, " IEEE Transactions on Plasma Sci., 19(4), p. 660(1991); and Sobolewski, "Electrical characterization of radio-frequency discharges in the Gaseous . . . ", J. Vac. Sci. Technol., 10(6) (1992). For real-time control of etching processes using multivariate statistical analysis, see Fox & Kappuswamy (U.S. Pat. No. 5,479,340).
In the prior art, typical monitoring and control systems suppress the occurrence of an arc by immediately shutting down the power input and, in some cases, activating a switch to discharge a voltage of opposite sign across the arcing electrodes to extinguish the arc. Examples include Teschner (U.S. Pat. No. 5,192,894), Anderson et al. (U.S. Pat. No. 5,241,152). Sturmer & Teschner (U.S. Pat. No. 5,281,321). Drummond (U.S. Pat. No. 5,427,669) and Lantsman (U.S. Pat. No. 5,584,972). However, in many cases, these patents disclose a method of response to an arc that may have already damaged the substrate or process hardware.